Refractory metals such as tantalum are important because their physical and/or chemical properties often make them the materials of choice for a variety of applications. For example, the chemical inertness of tantalum makes it a material of choice for use by industries employing corrosive chemicals such as in processes involving sulfuric acid, hydrochloric acid, hydrobromic acid, nitric acid, and chlorine. Tantalum is also used in the pharmaceutical industry for the manufacture of drugs, in the food industries for the processing of food products, and in the oil refining industry for petrochemical processing.
The physical and/or chemical properties of a refractory metal, such as the tensile strength and corrosion resistance, may improve by alloying with another metal or metals. In contrast, these properties may diminish in the presence of unwanted impurities to such a degree that the metal is useless for many important applications. Therefore, the purification of refractory metals is important.
The "Bessemer process" has been used for over 100 years in the purification of iron. It involves directing a blast of air through molten wrought iron (Fe) to produce steel, wherein the oxygen in the air reacts with impurities in the iron to produce volatile oxides and thus purify the iron. Refractory metals may not be suited to purification by the "Bessemer Process" since, unlike iron, refractory metals such as tantalum have much higher melting points and require a higher energy source for melting.
Two relatively recent and commercially important remelting techniques known as "electron beam remelting" (EBR) and "vacuum-arc remelting" (VAR), employ high energy sources and have been successfully used in the purification of refractory metals. Typically, the starting material is placed in a chamber with the energy-generating source, the beam for EBR or the arc for VAR, such that it lies in or can be directed into the path of the beam or arc. The chamber is evacuated, and the corresponding beam or arc is generated and caused to make contact with the starting material, whereupon the starting material rapidly becomes molten. As the melt is generated, it is collected in a cooled vessel such as a cooled copper crucible from which it is recovered. Although excellent results have been obtained using EBR and VAR, both techniques require relatively pure starting materials. Low grade materials, i.e. metals having oxygen impurities above about 5000-10,000 PPM, are not purified by EBR and VAR because extensive outgassing from the material during melting rapidly decreases the vacuum level in the chamber required for sustaining the beam or arc.
Another melting technique known as "plasma arc melting" (PAM) has also been used to purify refractory metals. Instead of a beam or arc heat source, a high temperature plasma is used to melt a refractory metal. This plasma can be generated using a plasma arc torch. For example, see U.S. Pat. No. 5,239,162 by R. E. Haun et al. entitled "Arc Plasma Torch Having Tapered-Bore Electrode," which issued Aug. 24, 1993. In the '162 patent, the operation of the torch is described in column 5, lines 24-33. An electric power source is activated to generate a potential between an electrode and a pool which contains metal, thereby generating an arc between them. Gas is injected into a chamber by the electrode and forced through a nozzle toward the pool. The arc ionizes the gas into a hot plasma gas, which blows against the pool and melts the metal in the pool. Importantly, since PAM is performed under gas pressure rather than under vacuum, the plasma formed is unaffected by outgassing during melting. In contrast to EBR and VAR, PAM has been used in the purification of low grade refractory metals. For a review of plasma metallurgy and plasma melting, which includes plasma arc melting and remelting, see "Recent Developments in Plasma Metal Processing" by K. Mimura et al. in High Temperature Materials and Processes, Vol. 7, No. 1, 1986, pages 1-16.
Generally, PAM involves the exposure of an impure metal to a plasma generated from gases such as argon (Ar) and argon/helium (Ar/He). The plasma formed from these gases may have a temperature range which includes 5,000-10,000.degree. C. Any known metal subjected to a plasma in this temperature range becomes molten. If the chemical composition of the starting impure metal is known, and if it assumed that this composition approximates that of the melt, a purification procedure can be formulated by examining the possible chemical reactions between components in the melt and determining which reactions are thermodynamically favorable under the appropriate conditions. For example, it is known that when carbon is exposed to oxygen at high temperatures, a thermodymically favorable chemical reaction takes place resulting in the formation of carbon monoxide. If a refractory metal having oxygen and carbon impurities is subjected to a high temperature Ar/He plasma, the metal becomes molten and the carbon and the oxygen impurities within the melt combine to produce carbon monoxide. This reaction is known as the carbothermal reaction, and is an efficient means for removing oxygen and carbon from the melt. In addition, oxygen can also be removed from the melt at high enough temperatures by direct volatilization in the absence of carbon.
The removal of oxygen from a metal by PAM may be further enhanced by employing plasmas having high thermal conductivities. For example, the addition of another chemical species to an Ar or Ar/He plasma may increase the thermal conductivity of the resulting plasma. In particular, an Ar/He/H.sub.2 plasma has a higher thermal conductivity than an Ar/He plasma, and its higher thermal conductivity results in a more efficient transfer of heat from the plasma to the metal.
Compared to an Ar or an Ar/He plasma, an Ar/He/H.sub.2 plasma may also provide additional mechanisms for oxygen removal from the metal. One such mechanism involves the chemical reaction between hydrogen and oxygen within the metal to produce species such as hydroxide (OH) radicals and water molecules (H.sub.2 O). Support for this mechanism is found in C. V. Robino, "Representation of Mixed Gases on Free Energy (Ellingham-Richardson) Diagrams," by Metallurgical and Material Transactions B, volume 27B (1996) pages 65-69. The Ellingham-Richardson diagram on p. 68 shows a plot of the energy vs. temperature for a variety of oxidation reactions. Each reaction is represented by a single curve in the diagram. Included in the diagram is a curve for the reaction of H.sub.2 with oxygen, and a curve for the reaction of H with oxygen. The curve for the former reaction lies at higher energy as compared to the curve for latter, indicating that the latter reaction is thermodynamically more favorable than the former. The diagram also includes a curve for the reaction of carbon with oxygen, i.e. the carbothermal reaction. At temperatures greater than about 2000.degree. C., the position of this curve indicates that the carbothermal reaction is thermodynamically more favorable than the reaction of oxygen with either H.sub.2 or H. Therefore, oxygen present in a melt will preferentially react with carbon when exposed to a hydrogen-containing plasma.
A process employing an Ar/He/H.sub.2 plasma for processing metal powder into metal flakes is described et al. in U.S. Pat. No. 4,743,297 by Kopatz et al. entitled "Process For Producing Metal Flakes", which issued on May 10, 1988. This process includes the steps of melting metal powder particles in flight in the plasma, impinging the melt on a rapidly spinning cold disc to produce flakes, and applying a jet of non-oxidizing gas to remove the solidified flakes from the disc. Although the '297 Patent mentions in column 2, lines 9-11 that "if necessary, the starting powders are exposed to temperatures and controlled environment to remove carbon, oxygen, etc.," the conditions for removal of carbon, oxygen, etc. from specific metals are not described.
The reduction of tantalum oxide (Ta.sub.2 O.sub.5) by PAM is described by K. Mimura et al. in "Production of Pure Tantalum by Carbon-Reduction Smelting and Hydrogen Plasma-Arc Melting with Refining," by Met. Trans., JOM, 31, 10, 1990, pages 293-301. Mimura et al. reduce Ta.sub.2 O.sub.5 to tantalum metal (Ta) by mixing Ta.sub.2 O.sub.5 with carbon and subjecting the mixture to an Ar or Ar/H.sub.2 plasma, wherein the carbon reacts with the oxygen to form carbon monoxide, leaving tantalum metal in highly purified form. Mimura et al. describe the mixing of graphite powder with Ta.sub.2 O.sub.5 powder at mole ratios between 4.50-5.50, pressing the powder into pellets, preheating the pellets to prevent scattering of the pellets during smelting, smelting the pellets with either an Ar plasma or an Ar/H.sub.2 plasma, and comparing the effects of a chosen plasma composition to the corresponding chemical composition of the purified tantalum-containing products. Mimura et al. generally employ a melt rate of about 1 gram/min. Mimura et al. state that the final product composition is dependent on the initial C/Ta.sub.2 O.sub.5 ratio. For example, a C/Ta.sub.2 O.sub.5 ratio of 5.10 gives a button-like product with metallic luster, whereas a C/Ta.sub.2 O.sub.5 ratio of 4.50 gives a flat plate with a black surface. As the C/Ta.sub.2 O.sub.5 ratio is increased, the amount of carbon in the final product also increases while the amount of oxygen in the final product decreases.
Chemical extraction techniques have successfully been used for the purification of refractory metals. However, chemical extraction is laborious and generally results in the production of unwanted chemical waste by-products.
In view of the current attitude regarding the conservation and optimum use of natural resources, the commercial recycling of spent metals is important. Therefore, an object of the present invention is to provide a process for removing carbon and oxygen impurities from tantalum.
Another object of the invention is to provide a process to convert low grade tantalum to a purity suitable for further processing by EBR and VAR.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.